26 http://www.yachtingmonthly.com November 2017
EXPERT ON BOARD How to unlock tHe secrets of winds on a weatHer sHore
Cold seaInshore recirculation of air within a cool boundary layerCool air Onshore
breezeFluky
breezeWarm wind
off the landClouds passing overheadDiagram 10Cold seaSea breezeRising Warm air Cool air
thermalsLand warmed by sunBackground wind
blowing offshore Sea breeze
Cumulus cloud frontSea breeze CalmDiagram 11Another thermal effect has been
described by meteorologist Mike Brettle,
who identified it on England’s East Coast.
By using data from offshore weather
stations, he demonstrated that when a
warm wind blows off a weather shore
on to a cold sea, the air close to the sea
may cool and become slower-moving,
which can cause the wind direction to
back rather than veer – contrary to the
advice in most sailing manuals, which
assume that the wind increases speed
as it crosses the coast and is no longer
slowed by the drag of land features. This
cooling effect can actually do more than
swing the wind. If the warm wind pulls at
the upper surface of the cool air, that can
even reverse the surface wind direction
by creating a re-circulating effect in which
the cool air then forms an on-shore
breeze (Diagram 10 and Photo 17). This
resembles a sea breeze but is generally
rather weak, particularly if the coastal
cliffs are higher than the cool layer,
because that dense air will then remain
on their seaward side. Usually, clouds that
are in the upper airstream continue to
drift out over the sea, unaffected by the
reversal at sea level.
A proper, classic sea breeze is more
likely where the coast is low-lying, so
that the cool air can then flow inland
and gather speed (Diagram 11). Cumulus
clouds, created by warm thermals of
rising air, are particularly intense above
the leading edge of the breeze, forming
what is known as a sea breeze front.
On the seaward side of that front the
thermals are cut off and over the coastal
strip there may well be clear blue sky.
The sea breeze can also be explained in
terms of atmospheric pressure, because
the process starts with the sun warming
the land and the air above it. As the
warmed air becomes less dense and
induces a reduction in pressure, this has
the effect of pulling in the denser, cool air
from over the sea (Photo 19). The most
powerful breezes occur where prolonged
warming of the air over a flat coastal
Cool power
17Over the Antrim hills,
the wind is blowing off
the land but there is a
faint on-shore wind
close to the cliffsFurther reaDing
n Coastal Turmoil, by Ken Endean.
Adlard Coles
n Weather to Sail, by Mike Brettle and
Bridget Smith. The Crowood Press Ltd
n Sea Breeze and Local Wind, by J E
Simpson. Cambridge University PressIn this sea breeze, the Brittany coast is
a friendly lee shoreplain causes a widespread reduction in
pressure, which has the same effect as a
small depression. It can generate quite a
serious wind, reaching Force 5 or more in
ideal conditions. However, the peak wind
strength only persists for a short period,
generally around mid-afternoon, and the
waves are unlikely to grow large enough
to cause trouble. Sailing conditions
are usually delightful (Photo 18), with
glittering waves on a blue sea and plenty
of sunshine. Of course, the sea breeze has
turned a weather shore into a lee shore,
but it is a friendly lee shore. WA splendid sea breeze: glittering blue
sea, warm sun and small waves19 18PHOTOS: KEN ENDEAN